Copolymers of this type were manufactured according to the process described in document EP 1 748 053. This process is carried out in an autoclave and in the presence of an initiator for initiating the polymerization reaction between the monomers.
The process described in the above document thus relates to the manufacture of copolymers comprising at least three monomers including vinylidene difluoride (VDF) and trifluoroethylene (TrFE) combined with at least one other monomer that is more reactive than the latter monomers.
This process comprises the following steps:    a) feeding a mixture of the three constituent monomers into a predefined reaction volume, said mixture being poor in reactive monomer,    b) as soon as the reaction is initiated, continuous reinjection at a constant pressure of a mixture of the constituent monomers, richer in the most reactive monomer, the composition of said mixture being determined in a known manner by calculation.
The tests carried out by the applicant made it possible to observe that this process had a certain number of drawbacks.
Specifically, it was observed that the copolymers obtained had properties which were sometimes not reproducible, since the reinjection carried out in the second step of the process only took place after the initiation of the polymerization reaction. However, it was observed that the duration of the initiation phase of the reaction was often imprecise, or even random.
Furthermore, it was observed that the presence, in the mixture of the initial feed, of one monomer that was more reactive than the other monomers, such as CFE, had a tendency to slow down the initiation phase of the reaction over time.
Moreover, it was observed that the implementation of the polymerization according to the process above resulted in a high fouling of the reactor, with the presence of crusts and skins both on the walls of the reactor and within the copolymer powder obtained. This phenomenon is even more pronounced when the polymerization temperature is high, which limits the possibilities of obtaining polymers that have very high molecular weights. This fouling of the reactor, both of its walls and within the powder, is evaluated on a scale having 4 levels numbered from 0 to 3 and characterized as follows:                Level 0:                    reactor clean after emptying and simple rinsing with a water jet,            resin free of any crust or skin.                        Level 1:                    presence of a friable coating on a minor portion of the reactor walls,            slight film or coating on the stirrer spindle,            clean thermometer well,            powder with no or with very few aggregates or lumps.                        Level 2:                    presence of a not very friable, or even hard, coating on a major portion of the reactor walls,            fouled or encrusted stirrer spindle,            film on the thermometer well,            aggregates and bits of crust in the powder                        Level 3:                    presence of a thick resistant film on the major portion of the surface of the reactor,            highly encrusted stirrer spindle,            blocks and filaments around the thermometer well,            presence of numerous skins and crusts in the powder.                        
By using this scale for evaluating the fouling level after polymerization, level 1, 2 or 3 is attributed to polymerizations carried out according to the above process, whereas the process described in the present text makes it possible to regularly and reproducibly obtain level 0.